Physics quiz question: speed at highest point in trajectory

Physics 205A Quiz 3, Fall Semester 2009
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Problems 3.20, 3.49(b)

A softball is thrown with an initial velocity of 15 m/s at angle of 20° above the horizontal. Neglect air resistance. What is the speed of the softball at the highest point in its trajectory?
(A) 0 m/s.
(B) 5.1 m/s.
(C) 14 m/s.
(D) 20 m/s.

Correct answer: (C)

The initial horizontal and vertical velocity components are v_ix = 14 m/s and v_iy = 5.1 m/s, respectively. The vertical velocity will continuously change throughout the trajectory, and vanish at the highest point. However, the horizontal velocity will remain constant throughout the trajectory, and thus be the sole contribution to the speed at the highest point. Response (D) is 2*a_y, where a_y = 9.8 m/s^2.

Student responses
Sections 70854, 70855
(A) : 14 students
(B) : 10 students
(C) : 23 students
(D) : 1 student

Success level: 47%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.63

Student responses
Section 72177
(A) : 4 students
(B) : 10 students
(C) : 1 student
(D) : 0 students

Success level: 66%
Discrimination index (Aubrecht & Aubrecht, 1983): 1.00